Digital current regulator

ABSTRACT

A digital current regulator for controlling the operation of an inductance device such as an AC motor. A plurality of independently controllable insulated gate bipolar transistors are selectively enabled to vary the instantaneous phase and magnitude of current applied to an AC motor. To minimize induced back EMFs during current switching operations, a number of safeguards are utilized. Switching between adjacent states only is provided to minimize the magnitude of instantaneous current changes. A &#34;ready blocking&#34; function limits the maximum switching frequency to avoid significantly derating the switching device capabilities, while a &#34;time out&#34; feature assures system operation above at least a minimum switching frequency to further minimize the magnitude of instantaneous current changes. A time hysteresis function is provided to avoid switching device response to false feedback control signals.

BACKGROUND OF THE INVENTION

This invention relates generally to current regulators and, moreparticularly, to current regulators for regulating current in linear orrotating alternating current machines.

Various types of electric motors, such as direct current (DC) andalternating current (AC) motors, are in existence. Although each type ofmotor has some unique characteristic that renders it suitable for aparticular task, each type also has its drawbacks and no one type ofmotor is suitable for all applications.

DC motors are very useful and versatile because, with such motors, it isvery easy to independently control motor speed and torque. However, DCmotors require the use of a commutator and its associated brushes. Thesemechanical elements increase motor cost and complexity, reduce overallruggedness and reliability and require frequent maintenance.

AC motors, on the other hand, are simpler and more economical inconstruction, and are more rugged and reliable in operation, than DCmotors. However, AC motors are more sluggish in operation than their DCcounterparts. Furthermore, it is difficult to control motor speed andtorque independently in AC motors. These drawbacks sometime outweigh theadvantages of AC motors in certain applications.

In an effort to obtain the operational advantages of DC motors togetherwith the mechanical and economic advantages of AC motors, variouscontrollers for AC motors have been developed. Typically, thesecontrollers have current regulators that vary the instantaneous currentto the AC motor in response to a command so as to achieve a desiredcharacteristic in motor operation. In one type of current regulator, aplurality of electronic power switches are coupled to a source ofelectrical current. A control circuit selectively enables various onesof the electronic switches to change the instantaneous phase andmagnitude of the current applied to the AC motor. By properlycontrolling the phase and magnitude of the motor current, it is possibleto control such operational characteristics as torque and motor speedsomewhat independently. In this manner, it is possible to mimic DC motoroperation using an AC motor. However, because of the large inputinductance presented by an AC motor, any abrupt or large magnitudechanges in motor current can result in large induced countervoltages or"back EMFs." Unless carefully limited, such back EMFs can destroy theelectronic switching devices.

To minimize the magnitude of undesirable back EMFs, some currentregulators employ "soft switching" wherein the states of the switchingdevices are only changed when the instantaneous motor current is nearzero. Although this avoids the production of large induced back EMFs,soft switching greatly reduces the maximum effective switching frequencyand drastically limits the range of motor response characteristics thatcan be achieved with "soft switching" systems. "Hard switching" on theother hand, which permits the switching devices to change state evenwhen motor current is non-zero, provides a greater range of achievablemotor response characteristics. However, because hard switching canresult in abrupt current changes and consequently large induced backEMFs, hard switching is difficult to implement in actual practice.

Typically, current regulators are "load specific" in that they aretailored to (and only work with) motors having specific electrical inputspecifications. Such regulators are difficult and time consuming toadapt for use with any motor other than the one for which they aredesigned.

A need exists, therefore, for a switching current regulator that canprovide the versatility and responsiveness of a "hard switching" systemwith the reliability and durability of a "soft switching" system. Such acurrent regulator working in conjunction with a controller will betterenable an AC motor to match the operational characteristics of a DCmotor and thereby enhance the versatility of otherwise rugged andreliable AC motors. By making a current regulator "load independent"(i.e., operational with a variety of motors having varying electricalinput specifications) system versatility can be further enhanced.

In view of the foregoing, it is a general object of the presentinvention to provide a new and improved current regulator for inductiondevices such as AC motors.

It is a further object of the present invention to provide a new andimproved current regulator that provides a wide range of attainable,motor operational response characteristics

It is a further object of the present invention to provide a new andimproved current regulator that is substantially load independent.

It is a further object of the present invention to provide a new andimproved current regulator that permits enhanced and substantiallyindependent control of such AC motor operational characteristics astorque and speed.

It is a further object of the present invention to provide a new andimproved current regulator that permits hard switching to furtherincrease system versatility and effectiveness.

SUMMARY OF THE INVENTION

The invention provides a current regulator for controlling analternating current machine device. The current regulator includes anelectrical current source and a plurality of electronic switchingelements coupled between the current source and the alternating currentmachine. Each of the switching elements is controllably switchablebetween a conductive state and a nonconductive state. The currentregulator further includes a control circuit coupled to the electronicswitching elements and operable to switch selected ones of theelectronic switching elements between the conductive and nonconductivestates so as to achieve a desired current in the induction device. Thecontrol circuit includes structure operatively associated with theelectronic switching elements for insuring operation of the currentregulator at a switching frequency at or above a predetermined lowfrequency threshold. Additional structure operatively associated withthe electronic switching elements ensures operation of the currentregulator at a switching frequency at or below a predetermined highfrequency threshold. Still additional structure, associated with theelectronic switching elements and responsive to each transition of theelectronic switching elements between the conductive and nonconductivestates, maintains the electronic switching elements in the respectivestates for a predetermined minimum time period following each suchtransition.

The invention also provides a current regulator for regulating currentfrom a current source to an alternating current machine so as toapproximate a desired current in the machine device and thereby obtain adesired predetermined operational characteristic from the machine. Thecurrent regulator includes a plurality of electronic switching elementscouplable to the current source of and to the machine. The electronicswitching elements are selectively actuable to predictably alter theinstantaneous current supplied to the machine. A current error circuit,responsive to the instantaneous current supplied to the machine,develops an error signal indicative of the degree by which theinstantaneous current supplied to the machine deviates from the desiredcurrent. A processor circuit, responsive to the current error signal,actuates selected ones of the electronic switching elements as needed tocause the instantaneous current to more closely approximate the desiredcurrent. The processor circuit includes a switching frequency controlcircuit that functions to ensure that the switching frequency of theelectronic control elements is maintained between predetermined upperand lower frequency thresholds. A current change limit circuit functionsto ensure that the maximum rate of the change of the instantaneouscurrent is kept below a predetermined maximum threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The invention,together with the further objects and advantages thereof, may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings, wherein like referencenumerals identify like elements, and wherein:

FIG. 1 is a functional block diagram of an AC motor control systemincluding a digital current regulator embodying various features of theinvention.

FIG. 2 is a conceptual representation of the possible switching statesfor the AC motor control system shown in FIG. 1.

FIG. 3 is a table of the possible switching states available for the ACmotor control system.

FIG. 4 is a table useful in understanding the concept of "adjacent" nextstates.

FIG. 5 is a table useful in understanding the concept of "closest" and"farthest" zero states.

FIG. 6 is a diagram showing the permissible state transitions providedby the AC motor control system of FIG. 1.

FIG. 7 is a functional block diagram of the current regulatorincorporated in the motor control system shown in FIG. 1.

FIG. 8 is a logic flow diagram implemented in one embodiment of adigital current regulator embodying various features of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, and, more particularly, to FIG. 1, a controlsystem 10 for controlling the operation of an alternating currentmachine is shown in functional block diagram form. In the illustratedexample, the AC machine is a three phase AC servo motor 12. The controlsystem 10 provides the three phase power needed to operate the servomotor 12 and functions, broadly, to vary the magnitude and phase of thepower supplied to the motor 12 so as to achieve a desired motor responseor velocity profile.

Referring further to FIG. I, the system 10 includes an operator commandinput, output and communication unit 14 through which system commandsare received. A command interface module 16 accepts commands from theoperator command input 14 and converts the commands to digital form forfurther processing by the control system 10. Similarly, the commandinterface module 16 returns system operating information to the operatoror to other processors that may be associated with the control system 10or the AC servo motor 12.

The control system 10 further includes a controller module 18 thataccepts commands from either the command interface module 16 or anexternal operator keypad 20. The controller module 18 responds tooperator inputs received from either the command interface module 16 orthe keypad 20 and determines, in known manner, what current should beapplied to the AC servo motor 12 to achieve the desired motor response.To this end, the controller module 18 includes a preprogrammed "motionprofile" shaping circuit 22 that permits the operator to select any oneof a number of desired, preprogrammed, motor velocity profiles. Inaddition, the controller module 18 further includes a proportionalintegral/derivative (PID) speed and position control 24 and a vectorcontrol 26 that, together with a motion feedback signal derived from afeedback transducer 28 associated with the AC servo motor 12, generatecurrent command instructions as needed to cause the actual AC servomotor velocity profile to approximate the preprogrammed velocity profilespecified by the motion profile shaping circuit 22. In other words, thecontroller module 18 monitors the actual velocity profile of the ACservo motor 12, compares the actual velocity profile to the desiredvelocity profile specified by the motion profile shaping circuit 22, andthereafter specifies whatever changes in motor current are needed tocause the motor 12 to more closely follow the desired, preprogrammedvelocity profile.

The current command instructions generated by the controller module 18are applied to a digital current regulator module 30 embodying variousfeatures of the invention. The digital current regulator module 30responds to the current commands generated by the controller module 18and, in turn, develops a series of drive signals that are appliedthrough a gate driver module 32 to a switching power section 34 of knownconstruction. The power switching section, which, in the illustratedembodiment, includes a plurality of insulated gate bi-polar transistors(IGBT) coupled to a source of electrical power, responds to the drivesignals developed by the digital current regulator module 30 and, inresponse to these drive signals, controls the magnitude and phase ofpower applied to the AC servo motor 12.

It will be appreciated that, for any number of reasons, the actual,instantaneous velocity of the AC servo motor 12 may deviatesubstantially from the desired velocity specified by the motion profileshaping circuit 22. In such an event, the current commands developed bythe vector control circuit 26 of the controller module 18 may call forrapid current changes that, if actually implemented, could induce backEMF's sufficient to destroy the switching devices in the IGBT switchingpower section 34. To avoid such difficulties, the digital currentregulator 30, as will be described below, controls the switching of thepower section switch devices so as to implement the current commands asrapidly and faithfully as possible consistent with avoiding potentiallydestructive back EMF's. In this manner, the digital current regulator 30of the present invention increases the versatility and utility of ACmachines, such as AC motors, by expanding the range of velocity profilesattainable by such devices.

To understand the construction and operation of the digital currentregulator, it is first desirable to clarify terminology. The "state" ofthe switching power section can be visualized with reference to FIGS. 2and 3. FIG. 2 is a conceptual representation of the switching powersection 34. As is well known in the art, a switching power sectionintended for use in, for example, a three phase system will include sixseparate switch devices arranged in three independently controllablepairs. Each independently controllable pair of switch devices isrepresented conceptually by one of the single pole double throw"switches" 36, 36', 36" shown in each of the state diagrams of FIG. 2.With three independently controllable "switches," 36, 36', 36" there areeight possible switching states "K" labeled K=0 through K=7. Thesestates are further illustrated in the state table of FIG. 3.

In FIG. 3, the device states for each of the three independentlycontrollable "switches" 36, 36', 36" are shown in the right hand columnwith the number "1" corresponding to the switch being in the "up"position (FIG. 2) and with the number "0" corresponding to the switchbeing in the down position. It will be appreciated by those skilled inthe art that the representations of FIG. 2 and 3 are a conceptual,rather than literal, depiction of the actual switching devices in theswitching power section 34.

Referring further to FIGS. 2 and 3, it will be appreciated that when thestates of all three "switches" 36, 36', 36" are the same (i.e., "111"(K=0) or "000" (K=7)) the current applied to the AC servo motor 12 iszero. It will also be appreciated that, because the K=0 and K=7 statesare output equivalent (i.e., both result in zero current to the motor12) there are two available "zero" states and six available "non-zero"states.

The concept of "adjacent" states can be understood by reference to FIG.4. Two states are considered "adjacent" so long as only one switchtransition needs be made to switch between states. For example, andreferring to FIG. 3, if the current state is "001," the adjacent statesare "011" or "101." Each of the adjacent states can be reached throughonly one additional switch transition. The state "111" would not be anadjacent state because two additional switch transitions would be neededto reach this state from "001." It should be noted that one of the zerostates (i.e., "000" or "111" ) is always an adjacent state no matterwhat the current state is. In other words, it is always possible toreach one of the zero states with a single switch transition.

The concept of "closest zero" and "farthest zero" can be understood byreference to FIG. 5. The "closest zero" is the zero state that can bereached through only one switch transition. The "farthest zero" is thezero state that can be reached through two switch transitions. Forexample, if the current state is "001," the "closest zero" is "000"because this state can be achieved through only one switch transition.On the other hand, the "farthest zero" is "111" because it requires twoswitch transitions to reach this state from the current state of "001."

In accordance with one aspect of the invention, the digital currentregulator 30 only permits transitions between adjacent states, includingthe closest and farthest zero states. This is illustrated in FIG. 6. InFIG. 6, the three phases of the power supplied to the AC servo motor aredesignated by the letters U, V and W. Each phase, in turn, can be ineither of two states corresponding to the state of the correspondingswitching devices controlling that phase. The particular state isindicated by the presence or absence of a negation bar "-" above thecorresponding phase designation letter. For example, the designation U VW designates one particular state, while U V W designates the state thatresults when each of the "switches" 36, 36', 36" is switched to itsopposite state. FIG. 6 depicts, therefore, the state transitions thatare possible when only those transitions between adjacent states arepermitted. For example, if the current state is U V W, the onlypermitted transitions are to the adjacent next states (U V W or U V W)or to the closest zero (U v W). A transition to another state, (e.g., UV W) which is neither an adjacent next state or a closest zero, is notpermitted.

In accordance with another aspect of the invention, various additionalmeans are provided for achieving rapid implementation of the currentcommands while avoiding the development of excessive back EMF's. Inparticular, time out means and ready-blocking means are provided forensuring that the switching devices of the switching power section 34are, at all times, switched at a switching frequency that lies betweenpredetermined low frequency and high frequency thresholds. Inparticular, the time out means, which will be described in greaterdetail below, functions to ensure operation of the current regulator 30at a switching frequency at or above the predetermined low frequencythreshold. Similarly, the ready-blocking means functions to ensureoperation of the current regulator 30 at a switching frequency at orbelow the predetermined high frequency threshold.

To further enhance implementation of the current commands while avoidingexcessive back EMF's, the current regulator 30, in accordance with stillanother aspect of the invention, includes time hysteressis means forholding each of the switching elements in a non-conductive or conductivestate for a predetermined minimum period of time or "minimum state time"following the transition of the switching element to the respectivestate. The minimum state time is determined empirically and is selectedso as to avoid the occurrence of state transition "jitters" that canoccur due to the non-ideal nature of the feedback currents provided bythe motion feedback transducer.

In general, the faster the switching frequency, the more accurately theactual motor current will reproduce the desired current and the moreclosely the actual motor speed profile will match the desired profile.However, as switching frequency is increased, thermal considerationsbecome significant and the power handling capabilities of the switchingelements are derated. As previously noted, the time out means and theready-blocking means function to maintain the switching frequencybetween predetermined upper and lower limits. This ensures accurateimplementation of the desired current while avoiding significantderating of the switch element power handling capabilities.

Implementation of the digital current regulator 30 embodying variousfeatures of the invention can best be understood by reference to FIG. 7.FIG. 7 is a simplified schematic representation, in terms of conceptualhard wired logic elements, of one possible implementation of the digitalcurrent regulator 30. It will be appreciated by those skilled in the artthat, in practice, the digital current regulator 30 can be implementedin the form a suitably programmed microprocessor-based system.

As shown in FIG. 7, the current regulator 30 includes a "next state"processor 38 of known construction for determining what the requested"next" state of the switching elements should be, based upon what errorsexist between the desired motor current and the actual motor current. Itwill be appreciated that, in a three phase device, if the actual currentin two of the phases is known, the current in the remaining phase can bededuced. Accordingly, the current regulator 30 derives three errorsignals, IU_(err) IV_(err) and IW_(err) from information provided withrespect to two of the phases, U and V. Signals indicative of the desiredcurrent, (IU_(cmd)) and the actual current (IU_(fbk)) existing in phaseU are applied to the inputs of a differential amplifier 40 thatdevelops, at its output, the error signal IU_(err) indicative of thecurrent error in phase U. Similarly, a second differential amplifier 42develops an error signal (IV_(err)) for phase V, while a thirddifferential amplifier 44 provides an error signal (IW_(err)) indicativeof current error in phase W. The error signals are applied to the nextstate processor 38 that, in known manner, determines what the next stateof the switching elements should be based on the various current errors.

Referring further to FIG. 7, the current regulator 30 further includesadditional processors 46, 48 that receive the requested next state and,based thereon, determine which of the two available zero statesconstitutes the farthest zero and the closest zero, respectively, to therequest next state. The farthest zero and the closest zero are appliedto the inputs of a first two channel multiplexer 50 that, in turn, iscontrolled by the output of a "device ready" flip-flop 52. The output ofthe device ready flip-flop 52 determines whether the farthest zero orthe closest zero appears at the output of the first multiplexer 50. Ifthe true or T output of the device ready flip-flop 52 is logic high, theoutput of the first multiplexer 50 will be the closest zero, while ifthe T output is logic low, the farthest zero will appear at the outputof the first multiplexer 50.

The output of the next state processor 38 is also applied to one inputof a second, two channel multiplexer 54, while the output of the firstmultiplexer 50 is applied to the remaining channel of the secondmultiplexer 54. The "select" input of the second multiplexer 54 isconnected to the "true" or T output of a second flip-flop 56 thatmonitors both the current state and the requested next state anddetermines whether the requested next state is adjacent to the currentstate or is one of the zero states. If the requested next state isadjacent to the current state, or if the requested next state is one ofthe two available zero states, a logic high appears at the T outputwhich is applied to the S input of the second multiplexer 54. When the Sinput of the second multiplexer 54 is high, (indicating that therequested next state constitutes a permissible transition) the output ofthe first multiplexer 50 appears at the output of the second multiplexer54. Otherwise, the requested next state appears at the output of thesecond multiplexer 54.

The output of the second multiplexer 54 is applied to one channel of athird two channel multiplexer 58. The requested next state is applied tothe remaining channel of the third two channel multiplexer 58, while theselect or S input of the third two channel multiplexer 58 is connectedto the T output of a third flip-flop 60. The third flip-flop 60 monitorsthe current state and provides a logic high T output in the event thecurrent state is one of the two possible zero states. If the currentstate is a zero state, the output of the second multiplexer 54 appearsat the output of the third multiplexer 58. If the current st is not azero state, the requested next state appears at the output of the thirdmultiplexer 58.

As further illustrated in FIG. 7, the digital current regulator 30includes two registers 62, 64 that store state information. One of theregisters 62 (the "current state" register) stores the state presentlyor currently being specified by the current regulator. The secondregister 64 (the "last state" register) stores the state that existedimmediately prior to the current state. When the contents of the currentstate register 62 are updated, the prior contents are loaded into thelast state register 64. Thus, the current state becomes the last stateand the next state then becomes the current state. The inverted andnon-inverted outputs of the current state register 62 form the commands,U, V, W and U, V, W, applied to the switch devices in the switchingpower section 34.

An additional flip-flop 66, connected to the output of the last stateregister 64, determines whether the last state is a zero state. An "and"gate 68 having inputs connected to the T output of the current statezero flip-flop 60 and the F output of the last state zero flip-flop 66provides a logic high output only when the current state is a zero stateand the last state is not a zero state. The output of the "and" gate 68triggers a "time out" timer 70 that provides a logic high at it Z outputa predetermined time after receiving the logic high trigger pulse fromthe output of the "and" gate 68.

The output of the last state register 64 and the output of the thirdmultiplexer 58 are applied to the inputs of an additional flip-flop 72that determines whether the output of the third multiplexer 58 is a zerostate or is adjacent to the last state. If so, a logic high appears atthe T output of the flip-flop 72. The T output of the flip-flop 72 andthe Z output of the time out timer 70 are applied to the inputs of an"or" gate 74, the output of which is applied to one input of another"and" gate 76. The remaining input of the "and" gate 76 is connected tothe T output of the current state zero flip-flop 60, and the output ofthe "and" gate 76 is connected to one input of an additional "or" gate78. The remaining input of the additional "or" gate 78 is connected tothe F output of the current state zero flip-flop 60. The output of theadditional "or" gate 78 is connected to one input of still another "and"gate 80. One remaining input of the additional "and" gate 80 isconnected to the T output of a "device ready" flip-flop 82 that providesa logic high at its T output when the switching devices are ready toaccept a further command. The remaining input of the additional "and"gate 80 is connected to the output of a "minimum state time" timer 84that, after the passage of a predetermined "minimum state time" periodfollowing triggering, provides a logic high pulse at its Z output. Theoutput of the "and" gate 80 is applied back to the current stateregister 62 and the last state register 64 and functions to control theupdating of each register. It will be appreciated that such updatingonly occurs when the "device ready" flip-flop 82 indicates that thedevices are ready to receive additional commands, the "minimum statetime" timer 84 indicates the passage of the predetermined "minimum statetime" period, and a logic high appears at the output of the additional"or" gate 78.

The operation of the digital current regulator system 30 can best beunderstood by reference to the flow diagram of FIG. 8. System operationbegins with determination and specification of the next state. Aspreviously noted, the "next" state 86 is determined based on the currenterrors existing at the moment. At this point, the next state isdetermined without any consideration to whether it is a zero state orwhether it is adjacent to the current state. Such consideration is madesubsequently.

Next, the system, at 88, determines whether the current state is one ofthe two possible zero states. If it is, and if the "time out" period setby the "time out" timer 84 has not yet elapsed (90), the system nextdetermines (92) whether the selected next state is the same as oradjacent to the last state. If it is, system operation progresses to thedecision point identified by reference numeral 94. This also occurs ifthe time out period has elapsed. If the time out period has not elapsed,and the next state is not the same as or adjacent to the last state(92), system operation progresses to the decision point identified byreference numeral 96.

If the current state is not a zero state (88), the system nextdetermines whether the next state is the same as or adjacent to thecurrent state (98). If it is, system operation progresses to thedecision point identified by reference numeral 94. If the current stateis not a zero state and is not the same as or adjacent to the currentstate, the system ignores the next state specified by the controllermodule and instead sets the next state to the closest zero of thecurrent state (100). If the switching devices are not ready to receivethe next state (102), the farthest zero of the current state is assignedto the next state, (104), after which system operation progresses to thedecision point 94.

At the decision point 94, the system determines whether the switchingdevices are ready to accept the next state. If they are not, systemoperation progresses to decision point 96. If they are, the system nextdetermines whether the "minimum state time" period established by theminimum state time timer has elapsed (106). If it has not, systemoperation reverts back to the beginning (86). In this manner, no statetransitions are permitted unless the "minimum state time" has elapsed.This ensures that each state is maintained for at least the "minimumstate time" period.

If the "minimum state time" period has elapsed, the current state andlast state registers are updated (108) and the minimum state time timeris reset (110). The current state becomes the last state, and the nextstate becomes the current state. After the current and last stateregisters are updated, the system next determines whether the currentstate is a zero state (96). If it is not, the system operationprogresses back to the beginning and the cycle is repeated using theupdated current and last states.

If the current state is a zero state, the system next determines whetherthe last state is a zero state (112). If it is not, the time out timeris triggered 114 and system operation remains in this loop until thetime out period has elapsed, after which system operation returns to thebeginning 86. If the last state is a zero state, system operationimmediately progresses back to the beginning.

It will be appreciated that the system operation provides numerousfeatures that permit rapid implementation of the current commands whileavoiding the development of excessive, induced back EMFs. First, thesystem will not permit state transitions except between adjacent states.In addition, the minimum state time timer assures that each selectedstate is maintained for at least the minimum state time period. Finally,the time out timer helps ensure that state transitions occur at at leasta predetermined minimum frequency.

In actual practice, the digital current regulator is preferablyimplemented in the form a suitably programmed microprocessor basedsystem incorporating the operational flow diagram of FIG. 8. Althoughthe system has been shown and described in the context of driving an ACservo motor, it will be appreciated that the system can be adapted foruse with various other forms of alternating current machinery anddevices. It will also be appreciated that, although the system has beenshown and described in conjunction with insulated gate bi-polartransistors, the system can be used with switching power sectionsemploying other types of switching devices.

While a particular embodiment of the invention has been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and, therefore, the aim in the appended claims isto cover all such changes and modifications as fall within the truespirit and scope of the invention.

We claim:
 1. A current regulator for controlling an alternating currentmachine, said current regulator comprising:a source of multiphasealternating current; a plurality of electronic switching elementscoupled between the source of multiphase alternating current and thealternating current machine, each of said switching elements beingcontrollably switchable between a conductive state and a nonconductivestate; and a control circuit coupled to said electronic switchingelements and operable to switch selected ones of said electronicswitching elements between said conductive and said nonconductive statesso as to achieve a desired current in said alternating current machine,said control circuit comprising:timeout means operatively associatedwith said electronic switching elements for ensuring operation of saidcurrent regulator at a switching frequency at or above a predeterminedlow frequency threshold; ready blocking means operatively associatedwith said electronic switching elements for ensuring operation of saidcurrent regulator at a switching frequency at or below a predeterminedhigh frequency threshold; and time hysteresis means operativelyassociated with said electronic switching elements and responsive toeach transition of said electronic switching elements between saidconductive and nonconductive states, and between said nonconductive andconductive states, for maintaining said electronic switching elements ineither of said nonconductive or conductive states for a predeterminedminimum period of time following the transition of said electronicswitching elements to respective ones of said conductive ornonconductive states.
 2. A current regulator as defined in claim 1wherein said electronic switching elements are capable of being switchedfrom a present state to one or more adjacent states and to one or morenon-adjacent states and wherein said current regulator further includeslockout means for inhibiting said electronic switching elements frombeing switched from an initial state to a non-adjacent state so thatswitching is permitted only between initial states and adjacent states.3. A current regulator as defined in claim 2 wherein said currentregulator further includes control means responsive to the current drawnby said alternating current machine for switching said electronicswitching elements among adjacent states only so as to achieve a desiredcurrent in said alternating current machine.
 4. A current regulator asdefined in claim 3 wherein said electronic switching elements compriseinsulated gate bipolar transistors.
 5. A current regulator as defined inclaim 4 wherein said alternating current machine comprises an AC motor.6. A current regulator for regulating current from a current source toan alternating current machine so as to approximate a desired current inthe alternating current machine and thereby obtain a desiredpredetermined operational characteristic from the induction device, saidalternating current regulator comprising:a plurality of electronicswitching elements couplable to the current source and to the inductiondevice, said electronic switching elements being selectively actuable topredictably change the instantaneous current supplied to the inductiondevice; current error means responsive to the instantaneous currentsupplied to the induction device for developing an error signalindicative of the degree by which the instantaneous current supplied tothe induction device deviates from the desired current; and controlmeans responsive to said current error signal for actuating selectedones of said electronic switching elements as needed to change saidinstantaneous current so as to more closely approximate the desiredcurrent, said control means including:frequency control means forpreventing state transitions of said electronic switching elementsexcept between preestablished upper and lower frequency limits; andcurrent change limit means for limiting the maximum rate of change ofsaid instantaneous current to below a predetermined threshold.
 7. Acurrent regulator as defined in claim 6 wherein said electronicswitching elements are switchable to adjacent and non-adjacent statesand wherein said current change limit means functions in part to preventthe switching of said electronic switching elements to said non-adjacentstates.
 8. A current regulator as defined in claim 7 wherein saidfrequency control means includes ready blocking means operativelyassociated with said electronic switching elements for ensuringoperation of said current regulator at a switching frequency at or belowa predetermined high frequency threshold.
 9. A current regulator asdefined in claim 8 wherein said frequency control means includes timehysteresis means operatively associated with said electronic switchingelements and responsive to each transition of said electronic switchingelements for maintaining each of said electronic switching elements in arespective state for a predetermined period of time following transitionof said electronic switching element to said state.
 10. A currentregulator as defined in claim 9 wherein said frequency control meansfurther includes timeout means operatively associated with saidelectronic switching elements for ensuring operation of said currentregulator at a switching frequency at or above a predetermined lowfrequency threshold.
 11. A current regulator as defined in claim 10wherein said electronic switching elements comprise insulated gatebipolar transistors.
 12. A current regulator as defined in claim 11wherein the alternating current machine is an AC motor.
 13. A currentregulator as defined in claim 11 wherein the alternating current machineis an AC induction motor.
 14. A current regulator as defined in claim 11wherein the alternating current machine is a synchronous motor.
 15. Acurrent regulator as defined in claim 11 wherein the alternating currentmachine is a brushless DC motor.